Electronic device including quantum dots

ABSTRACT

An electroluminescent device includes a first electrode and a second electrode facing each other, and an emissive layer disposed between the first electrode and the second electrode and including the quantum dots. The quantum dots include a semiconductor nanocrystal core including indium (In) and phosphorous (P), a first semiconductor nanocrystal shell disposed on the semiconductor nanocrystal core, the first semiconductor nanocrystal shell including zinc and selenium, and a second semiconductor nanocrystal shell disposed on the first semiconductor nanocrystal shell, the second semiconductor nanocrystal shell including zinc and sulfur, wherein the quantum dots do not include cadmium. The electroluminescent device has an external quantum efficiency of greater than or equal to about 9% and a maximum brightness of greater than or equal to about 10,000 candelas per square meter (cd/m 2 ).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2018-0102972 filed in the Korean IntellectualProperty Office on Aug. 30, 2018, and all the benefits accruingtherefrom under 35 U.S.C. § 119, the content of which in its entirety isherein incorporated by reference.

BACKGROUND 1. Field

Quantum dots and electronic devices (electroluminescent devices)including the same are disclosed.

2. Description of the Related Art

Quantum dots (e.g., nano-sized semiconductor nanocrystals) havingdifferent energy bandgaps may be obtained by controlling the sizes andcompositions of the quantum dots. In a colloidal synthesis, organicmaterials such as a dispersing agent may coordinate, e.g., be bound, toa surface of the semiconductor nanocrystal during the crystal growththereof, thereby providing a quantum dot having a controlled size andshowing, e.g., exhibiting, luminescent properties. The luminescentproperties of quantum dots may be used in various fields. For example,the quantum dots may be used in an electroluminescent device, aphotoluminescent device, or the like. Developing environmentallyfriendly quantum dots that may show, e.g., exhibit, enhanced luminousproperties without a toxic heavy metal (e.g., cadmium, mercury, lead, ora combination thereof) is desired.

SUMMARY

An embodiment provides an electroluminescent device including quantumdots having improved photoluminescence properties and enhancedstability.

An embodiment provides the quantum dots.

An embodiment provides a method of producing the quantum dots.

An embodiment provides a composition including the quantum dots.

According to an embodiment, an electroluminescent device includes,

a first electrode and a second electrode facing each other, and

an emissive layer disposed between the first electrode and the secondelectrode, the emissive layer including quantum dots,

wherein the quantum dots include a semiconductor nanocrystal coreincluding indium (In) and phosphorus (P), a first semiconductornanocrystal shell disposed on the semiconductor nanocrystal core, thefirst semiconductor nanocrystal shell including zinc and selenium, and asecond semiconductor nanocrystal shell disposed on the firstsemiconductor nanocrystal shell, the second semiconductor nanocrystalshell including zinc and sulfur,

wherein the quantum dots do not include cadmium,

wherein the electroluminescent device has an external quantum efficiencyof greater than or equal to about 9% and a maximum brightness of greaterthan or equal to about 10,000 candelas per square meter (cd/m²).

The quantum dots may have a maximum photoluminescent peak in a red lightwavelength region.

A wavelength of the maximum photoluminescent peak of the quantum dotsmay be greater than or equal to about 600 nanometers (nm) and less thanor equal to about 650 nm.

A difference between a maximum photoluminescent peak wavelength and afirst absorption peak wavelength of the quantum dots may be less than orequal to about 20 nm.

The quantum dots may have an average particle size of greater than orequal to about 8 nm.

The quantum dots may have an average particle size of greater than orequal to about 8.5 nm.

The quantum dots may have an average particle size of greater than about9 nm.

The quantum dots may have an average particle size of greater than about10 nm.

A quantum yield of the quantum dots may be greater than or equal toabout 60%.

A standard deviation of a particle size distribution of the quantum dotsmay be less than or equal to about 20% of an average size thereof. Astandard deviation of a particle size distribution of the quantum dotsmay be less than or equal to about 15% of an average size thereof.

In the quantum dots, a molar ratio of indium to a sum of sulfur andselenium (i.e., In:(Se+S), hereinafter which may be recited as the valueof In:(Se+S)) may be less than or equal to about 0.1:1.

In the quantum dots, a molar ratio of indium to a sum of sulfur andselenium may be less than or equal to about 0.05:1.

In the quantum dots, a molar ratio of indium to a sum of sulfur andselenium may be greater than or equal to about 0.02:1.

In the quantum dots, a molar ratio of sulfur to selenium (S:Se) may beless than or equal to about 4.5:1.

In the quantum dots, a molar ratio of sulfur to selenium may be greaterthan or equal to about 0.01:1.

In the quantum dots, a molar ratio of sulfur to selenium may be greaterthan or equal to about 0.11:1.

In the quantum dots, a molar ratio of zinc to indium may be greater thanor equal to about 10:1.

In the quantum dots, a molar ratio of zinc to indium may be greater thanor equal to about 15:1.

In the quantum dots, a molar ratio of zinc to indium may be less thanabout 52:1. In the quantum dots, a molar ratio of zinc to indium may beless than or equal to about 49:1.

The first semiconductor nanocrystal shell may be disposed directly on asurface of the semiconductor nanocrystal core.

The first semiconductor nanocrystal shell may not include sulfur.

A thickness of the first semiconductor nanocrystal shell may be greaterthan or equal to about 5 monolayers.

A thickness of the first semiconductor nanocrystal shell may be greaterthan or equal to about 6 monolayers.

A thickness of the first semiconductor nanocrystal shell may be lessthan or equal to about 15 monolayers.

The second semiconductor nanocrystal shell may be an outermost layer ofthe quantum dot.

The second semiconductor nanocrystal shell may be disposed directly on asurface of the first semiconductor nanocrystal shell.

The difference between the maximum photoluminescent peak wavelength andthe first absorption peak wavelength of the quantum dots may be lessthan or equal to about 20 nm (or less than or equal to about 17 nm orless than or equal to about 16 nm).

The electroluminescent device may include an electron transport layerbetween the second electrode and the emissive layer.

The electron transport layer may include a metal oxide comprising zinc.The metal oxide may be represented by Zn_(1-x)M_(x)O, wherein M is Mg,Ca, Zr, W, Li, Ti, Y, Al, or a combination thereof and 0≤x≤0.5.

In an embodiment, the aforementioned quantum dots included in theelectroluminescent device are provided.

The quantum dots include a semiconductor nanocrystal core includingindium (In) and phosphorous (P), a first semiconductor nanocrystal shelldisposed on the semiconductor nanocrystal core, first semiconductornanocrystal shell including zinc and selenium, and a secondsemiconductor nanocrystal shell disposed on the first semiconductornanocrystal shell, second semiconductor nanocrystal shell including zincand sulfur, wherein the quantum dots do not include cadmium, and whereina difference between a maximum photoluminescent peak wavelength and afirst absorption peak wavelength of the quantum dots is less than orequal to about 20 nm.

A wavelength of the maximum photoluminescent peak of the quantum dotsmay be greater than or equal to about 600 nm and less than or equal toabout 650 nm.

The quantum dots may have an average particle size of greater than orequal to about 8 nm.

The quantum dots may have an average particle size of greater than orequal to about 8.5 nm.

The quantum dots may have an average particle size of greater than about9 nm.

The quantum dots may have an average particle size of greater than about10 nm.

A standard deviation of a particle size distribution of the quantum dotsmay be less than or equal to about 20% of an average size thereof.

A quantum yield of the quantum dots may be greater than or equal toabout 60%. A full width at half maximum of the maximum photoluminescentpeak of the quantum dots may be less than or equal to about 40 nm.

The first semiconductor nanocrystal shell may be disposed directly on asurface of the semiconductor nanocrystal core, the first semiconductornanocrystal shell may not include sulfur, and the second semiconductornanocrystal shell may be an outermost layer of the quantum dot.

In the quantum dots, a molar ratio of indium to a sum of sulfur andselenium (i.e., In:(Se+S), hereinafter which may be recited as the valueof In:(Se+S)) may be less than 0.1:1.

In the quantum dots, a molar ratio of indium to a sum of sulfur andselenium may be greater than or equal to about 0.02:1.

In the quantum dots, a molar ratio of zinc to indium may be greater thanor equal to about 10:1.

In the quantum dots, a molar ratio of zinc to indium may be less thanabout 52:1.

A thickness of the first semiconductor nanocrystal shell may be greaterthan or equal to about 6 monolayers, greater than or equal to about 7monolayers, greater than or equal to about 8 monolayers, greater than orequal to about 9 monolayers, or greater than or equal to about 10monolayers.

The difference between the maximum photoluminescent peak wavelength andthe first absorption peak wavelength of the quantum dots may be lessthan or equal to about 17 nm (less than or equal to about 16 nm).

In other embodiment, quantum dots include:

a semiconductor nanocrystal core comprising indium and phosphorous,

a first semiconductor nanocrystal shell disposed on the semiconductornanocrystal core, the first semiconductor nanocrystal shell comprisingzinc and selenium, and

a second semiconductor nanocrystal shell disposed on the firstsemiconductor nanocrystal shell, the second semiconductor nanocrystalshell comprising zinc and sulfur,

wherein in the quantum dots, a molar ratio of indium to a sum of sulfurand selenium In:(Se+S) is greater than or equal to about 0.02:1 and lessthan or equal to about 0.1:1,

wherein the quantum dots do not include cadmium, and

wherein an average particle size of the quantum dots is greater than orequal to about 8 nanometers and less than or equal to about 50 nm.

In an embodiment, a composition that may include an organic solvent andthe aforementioned cadmium free quantum dots is provided.

The electroluminescent device of an embodiment is based on, e.g.,includes, environmentally friendly quantum dots and may show, e.g.,exhibit, enhanced electroluminescent properties (e.g., improved externalquantum efficiency (EQE) and increased maximum luminance) together withimproved stability.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages and features of this disclosure willbecome more apparent by describing in further detail exemplaryembodiments thereof with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic cross-sectional view of a quantum dot lightemitting diode (QD LED) device according to an embodiment.

FIG. 2 is a schematic cross-sectional view of a QD LED device accordingto an embodiment.

FIG. 3 is a schematic cross-sectional view of a QD LED device accordingto an embodiment.

FIG. 4 is an ultraviolet-visible (UV-Vis) absorption spectrum (a dottedline) and a photoluminescence spectrum (a solid line) of the quantumdots prepared in Example 1.

FIG. 5 is a graph of external quantum efficiency (EQE) (percent (%))versus voltage (volts (V)) for the device prepared in Example 4.

FIG. 6 is a graph of EQE (percent (%)) versus luminance (L) (candelasper square meter (cd/m²)) for the device prepared in Example 4.

DETAILED DESCRIPTION

Hereinafter, example embodiments of the present disclosure will bedescribed in detail so that a person skilled in the art would understandthe same. This disclosure may, however, be embodied in many differentforms and is not construed as limited to the example embodiments setforth herein.

In the drawings, the thickness of layers, films, panels, regions, etc.,are exaggerated for clarity. Like reference numerals designate likeelements throughout the specification. It will be understood that whenan element such as a layer, film, region, or substrate is referred to asbeing “on” another element, it can be directly on the other element orintervening elements may also be present. In contrast, when an elementis referred to as being “directly on” another element, there are nointervening elements present.

It will be understood that, although the terms “first,” “second,”“third” etc. may be used herein to describe various elements,components, regions, layers and/or sections, these elements, components,regions, layers and/or sections should not be limited by these terms.These terms are only used to distinguish one element, component, region,layer, or section from another element, component, region, layer, orsection. Thus, “a first element,” “component,” “region,” “layer,” or“section” discussed below could be termed a second element, component,region, layer, or section without departing from the teachings herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “At least one” is not to be construed as limiting “a” or“an.” “or” means “and/or.” As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.It will be further understood that the terms “comprises” and/or“comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

“About” as used herein is inclusive of the stated value and means withinan acceptable range of deviation for the particular value as determinedby one of ordinary skill in the art, considering the measurement inquestion and the error associated with measurement of the particularquantity (i.e., the limitations of the measurement system). For example,“about” can mean within one or more standard deviations, or within ±10%or 5% of the stated value.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Exemplary embodiments are described herein with reference to crosssection illustrations that are schematic illustrations of idealizedembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, embodiments described herein should not beconstrued as limited to the particular shapes of regions as illustratedherein but are to include deviations in shapes that result, for example,from manufacturing. For example, a region illustrated or described asflat may, typically, have rough and/or nonlinear features. Moreover,sharp angles that are illustrated may be rounded. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the precise shape of a region and are notintended to limit the scope of the present claims.

As used herein, a work function or an energy level (e.g., highestoccupied molecular orbital (HOMO) energy level or a lowest unoccupiedmolecular orbital (LUMO) energy level) is expressed as an absolute valuefrom a vacuum level. In addition, when the work function or the energylevel is referred to be “deep,” “high” or “large,” the work function orthe energy level has a large absolute value based on “0 eV” of thevacuum level, while when the work function or the energy level isreferred to be “shallow,” “low,” or “small,” the work function or energylevel has a small absolute value based on “0 eV” of the vacuum level.

As used herein, the term “Group” may refer to a group of Periodic Table.

As used herein, “Group I” may refer to Group IA and Group IB, andexamples may include Li, Na, K, Rb, and Cs, but are not limited thereto.

As used herein, “Group II” may refer to Group IIA and Group IIB, andexamples of Group II metal may be Cd, Zn, Hg, and Mg, but are notlimited thereto.

As used herein, “Group III” may refer to Group IIIA and Group IIIB, andexamples of Group III metal may be Al, In, Ga, and TI, but are notlimited thereto.

As used herein, “Group IV” may refer to Group IVA and Group IVB, andexamples of a Group IV metal may be Si, Ge, and Sn, but are not limitedthereto. As used herein, the term “metal” may include a semi-metal suchas Si.

As used herein, “Group V” may refer to Group VA, and examples mayinclude nitrogen, phosphorus, arsenic, antimony, and bismuth, but arenot limited thereto.

As used herein, “Group VI” may refer to Group VIA, and examples mayinclude sulfur, selenium, and tellurium, but are not limited thereto.

As used herein, when a definition is not otherwise provided,“substituted” refers to replacement of hydrogen of a compound, a group,or a moiety by a C1 to C30 alkyl group, a C2 to C30 alkenyl group, a C2to C30 alkynyl group, a C2 to C30 epoxy group, a C2 to C30 alkyl estergroup, a C3 to C30 alkenyl ester group (e.g., an acrylate group,methacrylate group), a C6 to C30 aryl group, a C7 to C30 alkylarylgroup, a 01 to C30 alkoxy group, a 01 to C30 heteroalkyl group, a C3 toC40 heteroaryl group, a C3 to C30 heteroalkylaryl group, a C3 to C30cycloalkyl group, a C3 to C15 cycloalkenyl group, a C6 to C30cycloalkynyl group, a C2 to C30 heterocycloalkyl group, a halogen (—F,—Cl, —Br, or —I), a hydroxy group (—OH), a nitro group (—NO₂), athiocyanate group (—SCN), a cyano group (—CN), an amino group (—NRR′wherein R and R′ are independently hydrogen or a C1 to C6 alkyl group),an azido group (—N₃), an amidino group (—O(═NH)NH₂), a hydrazino group(—NHNH₂), a hydrazono group (═N(NH₂)), an aldehyde group (—C(═O)H), acarbamoyl group (—C(O)NH₂), a thiol group (—SH), an ester group (RCOO—or —O(═O)OR, wherein R is a C1 to C6 alkyl group or a C6 to C12 arylgroup), a carboxyl group (—COOH) or a salt thereof (—C(═O)OM, wherein Mis an organic or inorganic cation), a sulfonic acid group (—SO₃H) or asalt thereof (—SO₃M, wherein M is an organic or inorganic cation), aphosphoric acid group (—PO₃H₂) or a salt thereof (—PO₃MH or —PO₃M₂,wherein M is an organic or inorganic cation), or a combination thereof.

As used herein, when a definition is not otherwise provided, the term“hydrocarbon” refers to a group including carbon and hydrogen (e.g.,alkyl, alkenyl, alkynyl, aryl, or the like). The hydrocarbon group maybe a group having a valence of at least one formed by a removal of atleast one hydrogen from an alkane, an alkene, an alkyne, an arene, orthe like. At least one methylene in the hydrocarbon group may bereplaced with an oxide moiety, a carbonyl moiety, an ester moiety, —NH—,or a combination thereof.

As used herein, when a definition is not otherwise provided, “aliphatic”refers to a C1 to C40 linear or branched hydrocarbon (e.g., alkyl,alkenyl, or alkynyl) group.

As used herein, when a definition is not otherwise provided, “alkoxy”refers to an alkyl group that is linked via an oxygen (i.e., alkyl-O—),for example, methoxy, ethoxy, and sec-butyloxy groups.

As used herein, when a definition is not otherwise provided, “alkyl”refers to a straight or branched chain, saturated, monovalent (e.g., C1to C40) hydrocarbon group (e.g., methyl or hexyl).

As used herein, when a definition is not otherwise provided, “alkylene”refers to a straight or branched saturated (e.g., C2 to C40) aliphatichydrocarbon group having at least two valences and optionallysubstituted with at least one substituent.

As used herein, when a definition is not otherwise provided, “alkynyl”refers to a straight or branched chain, monovalent (e.g., C2 to C40)hydrocarbon group having at least one carbon-carbon triple bond (e.g.,ethynyl).

As used herein, when a definition is not otherwise provided, “aromatic”refers to an organic compound or group comprising at least oneunsaturated cyclic group having delocalized pi electrons. The termencompasses both aromatic hydrocarbon compounds and heteroaromaticcompounds.

As used herein, when a definition is not otherwise provided, “aryl”refers to a hydrocarbon group having a valence of at least one, forexample, formed by the removal of at least one hydrogen atom from one ormore rings of an arene (e.g., phenyl or naphthyl).

As used herein, when a definition is not otherwise provided, the term“hetero” refers to inclusion of at least one (e.g., one to three)heteroatoms, where the heteroatom(s) may be N, O, S, Si, or P,preferably N, O, or S.

As used herein, when a definition is not otherwise provided, an “amine”group has the general formula —NRR, wherein each R is independentlyhydrogen, a C1-C40 alkyl group, a C7-C40 alkylaryl group, a C7-C40arylalkyl group, or a C6-C40 aryl group.

As used herein, when a definition is not otherwise provided, “arene”refers to a hydrocarbon having an aromatic ring, and includes monocyclicand polycyclic hydrocarbons wherein the additional ring(s) of thepolycyclic hydrocarbon may be aromatic or nonaromatic. Specific examplesof arenes include benzene, naphthalene, toluene, and xylene.

As used herein, when a definition is not otherwise provided, “arylalkyl”refers to a substituted or unsubstituted aryl group covalently linked toan alkyl group that is linked to a compound (e.g., a benzyl is a C7arylalkyl group).

As used herein, when a definition is not otherwise provided,“heteroaryl” refers to an aromatic group that comprises at least oneheteroatom covalently bonded to one or more carbon atoms in an aromaticring.

As used herein, an average size of particles (or quantum dots) may bedetermined by using an electron microscope analysis and optionally acommercially available image analysis program (Image J). The average maybe mean or median.

Hereinafter, a light emitting device according to an embodiment isdescribed with reference to drawings.

FIG. 1 is a schematic cross-sectional view of an electroluminescentdevice (hereinafter, also referred to as a light emitting device)according to an embodiment.

Referring to FIG. 1, a light emitting device 10 according to anembodiment includes a first electrode 11 and a second electrode 15facing each other, and an emissive layer 13 disposed between the firstelectrode 11 and the second electrode 15 and including quantum dots. Ahole auxiliary layer 12 may be disposed between the first electrode 11and the emissive layer 13. An electron auxiliary layer 14 may bedisposed between the second electrode 15 and the emissive layer 13.

The device may further include a substrate. The substrate may bedisposed on a major surface (e.g., lower surface) of the first electrode11 or on a major surface (e.g., upper surface) of the second electrode15. In an embodiment, the substrate may be disposed on a major surface(e.g., lower surface) of the first electrode. The substrate may be asubstrate including an insulation material (e.g., insulating transparentsubstrate). The substrate may include glass; a polymer such as apolyester (e.g., polyethylene terephthalate (PET), polyethylenenaphthalate (PEN)), a polycarbonate, a polyacrylate, a polyimide, apolyamideimide, or a combination thereof; a polysiloxane (e.g., PDMS);an inorganic material such as Al₂O₃, ZnO, or a combination thereof; or acombination thereof, but is not limited thereto. The substrate may bemade of a silicon wafer. Herein “transparent” may refer to a case wherethe substrate has a transmittance of greater than or equal to about 85%,for example, greater than or equal to about 88%, greater than or equalto about 90%, greater than or equal to about 95%, greater than or equalto about 97%, or greater than or equal to about 99% for light of apredetermined wavelength (e.g., light emitted from the quantum dots). Athickness of the substrate may be appropriately selected taking intoconsideration a substrate material but is not particularly limited. Thetransparent substrate may have flexibility. The substrate may beomitted.

One of the first electrode 11 and the second electrode 15 may be ananode and the other may be a cathode. For example, the first electrode11 may be an anode and the second electrode 15 may be a cathode.

The first electrode 11 may be made of a conductor, for example, a metal,a conductive metal oxide, or a combination thereof. The first electrode11 may be, for example, made of a metal or an alloy thereof such asnickel, platinum, vanadium, chromium, copper, zinc, or gold; aconductive metal oxide such as zinc oxide, indium oxide, tin oxide,indium tin oxide (ITO), indium zinc oxide (IZO), or fluorine doped tinoxide; or a combination of a metal and a metal oxide such as ZnO and Alor SnO₂ and Sb, but is not limited thereto. In an embodiment, the firstelectrode may include a transparent conductive metal oxide, for example,indium tin oxide. A work function of the first electrode may be higherthan a work function of the second electrode that will be describedlater. A work function of the first electrode may be lower than a workfunction of the second electrode.

The second electrode 15 may be made of a conductor, for example, ametal, a conductive metal oxide, a conductive polymer, or a combinationthereof. The second electrode 15 may be made of, for example, a metal oran alloy thereof such as aluminum, magnesium, calcium, sodium,potassium, titanium, indium, yttrium, lithium, gadolinium, silver, tin,lead, cesium, or barium; or a multi-layer structured material such asLiF/Al, Li₂O/Al, Liq/Al, LiF/Ca, or BaF₂/Ca, but is not limited thereto.The conductive metal oxide is the same as described above.

A work function of the first electrode may be higher than a workfunction of the second electrode. A work function of the first electrodemay be lower than a work function of the second electrode.

In an embodiment, a work function of the first electrode 11 may be fromabout 4.5 electronvolts (eV) to about 5.0 eV (e.g., from about 4.6 eV toabout 4.9 eV). The work function of the second electrode 15 may begreater than or equal to about 4.0 eV and less than about 4.5 eV (e.g.,from about 4.0 eV to about 4.3 eV).

In an embodiment, a work function of the second electrode 15 may be fromabout 4.5 eV to about 5.0 eV (e.g., from about 4.6 eV to about 4.9 eV).The work function of the first electrode 11 may be greater than or equalto about 4.0 eV and less than about 4.5 eV (e.g., from about 4.0 eV toabout 4.3 eV).

The first electrode 11, the second electrode 15, or a combinationthereof may be a light-transmitting electrode and the light-transmittingelectrode may be, for example, made of a conductive oxide such as a zincoxide, indium oxide, tin oxide, indium tin oxide (ITO), indium zincoxide (IZO), or fluorine doped tin oxide, or a metal thin layer of asingle layer or a multilayer. When one of the first electrode 11 and thesecond electrode 15 is a non-light-transmitting electrode, thenon-light-transmitting electrode may be made of, for example, an opaqueconductor such as aluminum (Al), silver (Ag), or gold (Au).

A thickness of the electrodes (the first electrode, the secondelectrode, or a combination thereof) is not particularly limited and maybe appropriately selected taking into consideration device efficiency.For example, the thickness of the electrodes may be greater than orequal to about 5 nm, for example, greater than or equal to about 50 nm.For example, the thickness of the electrodes may be less than or equalto about 100 micrometers (μm), for example, less than or equal to about10 μm, less than or equal to about 1 μm, less than or equal to about 900nm, less than or equal to about 500 nm, or less than or equal to about100 nm.

The emissive layer 13 includes (e.g., a plurality of) quantum dots. Thequantum dots (hereinafter, also referred to as semiconductornanocrystals) may absorb light from an excitation source to be excitedand may emit energy (a light of a wavelength) corresponding to an energybandgap of the quantum dots. The energy bandgap of the quantum dots mayvary with a size and a composition of the semiconductor nanocrystal. Forexample, as the size of a quantum dot increases, the quantum dot mayhave a narrower energy bandgap, thereby having an increased luminouswavelength. Semiconductor nanocrystals may be used as a light emittingmaterial in various fields such as a display device, an energy device,or a bio light emitting device.

Quantum dots having a photoluminescence (PL) property at an applicablelevel may include cadmium (Cd). Cadmium may cause environment/healthproblems and is one of the restricted elements via Restriction ofHazardous Substances Directive (RoHS) in many countries. A cadmium-freequantum dot may exhibit low electroluminescent properties and poorstability in comparison with a cadmium-based quantum dot. Accordingly,development of a cadmium-free semiconductor nanocrystal particle havingimproved photoluminescence characteristics (e.g., reduced full width athalf maximum (FWHM), enhanced EQE, increased luminance, or the like) andan electroluminescent device including the same are desired. Forexample, for the realization of a QD LED device (i.e., a display devicewithout a light source), a red light emitting and environmentallyfriendly quantum dot capable of exhibiting enhanced electroluminescentproperties is necessary. While an indium phosphide-based quantum dot isa type of cadmium-free and red light emitting quantum dot, an Indiumphosphide-based quantum dot may exhibit an insufficient level ofelectroluminescent properties. For instance, an indium phosphide-basedand red-light emitting quantum dot may exhibit an EQE of less than orequal to about 2.5%. Moreover, it may not be easy to improve thestability (e.g., the life time) of red-light emitting cadmium-freequantum dots.

The present inventors have found that the quantum dots having thefeatures descried herein may exhibit improved electroluminescentproperties. The present inventors have also found that the quantum dotsmay exhibit improved properties in an electroluminescent device (ifdesired when the quantum dots are combined with a zinc metal oxide-basedelectron transporting layer that will be described below). The quantumdots included in the device of an embodiment may have the size, thecomposition, the properties (e.g., the Stokes shift), or a combinationthereof that will be described below by controlling the preparationconditions (e.g., controlling the factors that may affect the growthrate of the core, shell, or a combination thereof such as a reactiontemperature, an injection rate of a precursor, types and amounts of theprecursor, or a combination thereof). When the quantum dots of anembodiment are combined with an inorganic electron auxiliary layer(e.g., an ETL) including a metal oxide comprising zinc such as a zincoxide, a zinc magnesium oxide, or the like, the quantum dots may exhibita high EQE and a high luminance (e.g., a EQE of greater than or equal toabout 9% and a maximum luminance of greater than or equal to about10,000 cd/m² that may be difficult to achieve for cadmium free quantumdots. For a red-light emitting electroluminescent device based oncadmium free quantum dots, increasing both of the EQE and the brightnessmay be desired. Moreover, the device including the quantum dots of anembodiment may exhibit improved stability.

Accordingly, an embodiment is related to the quantum dots (or apopulation of the quantum dots) that will be described in detail below.The quantum dots of an embodiment do not include cadmium. The quantumdots of an embodiment may not include a toxic heavy metal such ascadmium, mercury, lead, or a combination thereof.

The quantum dot includes a semiconductor nanocrystal core includingindium (In) and phosphorous (P), a first semiconductor nanocrystal shelldisposed on the semiconductor nanocrystal core and including zinc andselenium, and a second semiconductor nanocrystal shell disposed on thefirst semiconductor nanocrystal shell and including zinc and sulfur.

The quantum dots may have a maximum photoluminescent peak in a red lightwavelength region. The quantum dots may have a maximum photoluminescentpeak in a green light wavelength region.

When the quantum dots emit red light, a maximum photoluminescent peakwavelength of the quantum dots may be greater than or equal to about 600nm, greater than or equal to about 610 nm, greater than or equal toabout 611 nm, greater than or equal to about 612 nm, greater than orequal to about 613 nm, greater than or equal to about 614 nm, greaterthan or equal to about 615 nm, greater than or equal to about 616 nm,greater than or equal to about 617 nm, greater than or equal to about618 nm, greater than or equal to about 619 nm, or greater than or equalto about 620 nm. The maximum photoluminescent peak wavelength of thequantum dots may be less than or equal to about 650 nm, less than orequal to about 640 nm, less than or equal to about 635 nm, less than orequal to about 634 nm, less than or equal to about 633 nm, less than orequal to about 632 nm, less than or equal to about 631 nm, or less thanor equal to about 630 nm.

In a UV-Vis absorption spectrum of the quantum dots, the firstabsorption peak wavelength may be present in a range of greater thanabout 450 nm, greater than or equal to about 500 nm, greater than orequal to about 550 nm, or greater than or equal to about 570 nm, andless than the maximum photoluminescent peak wavelength (e.g., less thanor equal to about 620 nm, less than or equal to about 610 nm, less thanor equal to about 600 nm, or less than or equal to about 590 nm).

In an embodiment, the quantum dots may emit green light and the firstabsorption peak wavelength may be greater than or equal to about 480 nm,greater than or equal to about 485 nm, greater than or equal to about490 nm, greater than or equal to about 500 nm, greater than or equal toabout 510 nm, or greater than or equal to about 515 nm. In anembodiment, the quantum dots may emit green light and the firstabsorption peak wavelength may be, less than or equal to about 540 nm,less than or equal to about 530 nm, less than or equal to about 529 nm,less than or equal to about 528 nm, less than or equal to about 527 nm,less than or equal to about 526 nm, less than or equal to about 525 nm,less than or equal to about 524 nm, less than or equal to about 523 nm,less than or equal to about 522 nm, less than or equal to about 521 nm,or less than or equal to about 520 nm.

In an embodiment, the quantum dots may emit red light and the firstabsorption peak wavelength may be greater than or equal to about 595 nm,greater than or equal to about 596 nm, greater than or equal to about597 nm, greater than or equal to about 598 nm, greater than or equal toabout 599 nm, greater than or equal to about 600 nm, greater than orequal to about 601 nm, greater than or equal to about 602 nm, greaterthan or equal to about 603 nm, greater than or equal to about 604 nm,greater than or equal to about 605 nm, greater than or equal to about606 nm, greater than or equal to about 607 nm, greater than or equal toabout 608 nm, greater than or equal to about 609 nm, greater than orequal to about 610 nm, greater than or equal to about 611 nm, or greaterthan or equal to about 612 nm.

In an embodiment, the quantum dots may emit red light and the firstabsorption peak wavelength may be less than or equal to about 625 nm,less than or equal to about 624 nm, less than or equal to about 623 nm,less than or equal to about 622 nm, less than or equal to about 621 nm,less than or equal to about 620 nm, or less than or equal to about 619nm.

A difference between the wavelength of the maximum photoluminescent peakand the first absorption peak wavelength (hereinafter, also referred toas a Stokes shift) may be less than or equal to about 20 nm, less thanor equal to about 19 nm, less than or equal to about 18 nm, less than orequal to about 17 nm, or less than or equal to about 16 nm.

In a quantum dot, a Stokes shift refers to a difference between theabsorption energy and the photoluminescence energy. Thus, the Stokesshift may be represented by a difference between a first absorption peakwavelength (nm) (or energy, microelectronvolts (meV)) in a UV-Visabsorption spectrum and a maximum photoluminescence peak wavelength(energy) in a photoluminescence spectrum. As used herein, the “firstabsorption peak (or first excitation absorption peak)” refers to a mainpeak appearing first from a lower energy region in a UV-Vis absorptionspectrum. Without wishing to be bound any theory, it is believed thatthe Stokes shift may depend on a size, a composition, or a combinationthereof of quantum dot particles. Without wishing to be bound anytheory, it is also believed that a surface state of the quantum dots(e.g., presence of surface defect(s)) also may have an influence on theStokes shift. Thus, the Stokes shift of the quantum dots may represent asize, a composition, a surface state, or a combination thereof of thequantum dot. The present inventors have also found that the Stokes shiftof the core-shell quantum dots may have a direct effect on theelectroluminescent properties and the stability (e.g., T50 life time) ofthe device including the quantum dots.

The quantum dots may have an average particle size of greater than orequal to about 8 nm, greater than or equal to about 9 nm, or greaterthan or equal to about 10 nm. The average particle size of the quantumdots may be less than or equal to about 50 nm, less than or equal toabout 45 nm, less than or equal to about 40 nm, less than or equal toabout 39 nm, less than or equal to about 38 nm, less than or equal toabout 37 nm, less than or equal to about 36 nm, less than or equal toabout 35 nm, less than or equal to about 34 nm, less than or equal toabout 33 nm, less than or equal to about 32 nm, less than or equal toabout 31 nm, less than or equal to about 30 nm, less than or equal toabout 29 nm, less than or equal to about 28 nm, less than or equal toabout 27 nm, less than or equal to about 26 nm, less than or equal toabout 25 nm, less than or equal to about 24 nm, less than or equal toabout 23 nm, less than or equal to about 22 nm, less than or equal toabout 21 nm, less than or equal to about 20 nm, less than or equal toabout 19 nm, less than or equal to about 18 nm, less than or equal toabout 17 nm, less than or equal to about 16 nm, less than or equal toabout 15 nm, less than or equal to about 14 nm, or less than or equal toabout 13 nm. The particle size of the quantum dots may be a diameter ofthe particle. When the particle is non-spherical, the size of thequantum dots may be a diameter (e.g., equivalent diameter) convertedfrom an area of a two-dimensional image obtained from an electronmicroscope analysis into a circle.

A particle size distribution of the quantum dots may be less than orequal to about 20%, less than or equal to about 19%, less than or equalto about 18%, less than or equal to about 17%, less than or equal toabout 16%, or less than or equal to about 15% of the average sizethereof. In an embodiment, the particle size distribution of the quantumdots may be less than or equal to about 14%, less than or equal to about13%, less than or equal to about 12%, less than or equal to about 11%,or less than or equal to about 10% of the average particle size thereof.A particle size distribution of the quantum dots may be greater than orequal to about 5%, greater than or equal to about 6%, greater than orequal to about 7%, greater than or equal to about 8%, greater than orequal to about 9%, greater than or equal to about 10%, or greater thanor equal to about 11% of the average particle size thereof.

The quantum dots may have a quantum yield of greater than or equal toabout 60%, greater than or equal to about 65%, greater than or equal toabout 70%, greater than or equal to about 75%, or greater than or equalto about 80%.

In an embodiment, the semiconductor nanocrystal core may include anindium phosphide. The semiconductor nanocrystal core may further includezinc. The semiconductor nanocrystal core may not include zinc. A size ofthe core may be selected appropriately. In an embodiment, the size ofthe core may be greater than or equal to about 1 nm, greater than orequal to about 1.5 nm, greater than or equal to about 2 nm, or greaterthan or equal to about 2.5 nm. In an embodiment, the size of the coremay be less than or equal to about 5 nm, less than or equal to about 4nm, less than or equal to about 4.5 nm, less than or equal to about 3.5nm, or less than or equal to about 3 nm.

The first semiconductor nanocrystal shell may include a zinc selenide(e.g., ZnSe). The first semiconductor nanocrystal shell may not includesulfur (S). In an embodiment, the first semiconductor nanocrystal shellmay not include ZnSeS. The first semiconductor nanocrystal shell may bedisposed directly on the semiconductor nanocrystal core. The firstsemiconductor nanocrystal shell may have a thickness of greater thanabout 4 monolayers (ML), for example, greater than about 4.5 MLs,greater than or equal to about 5 MLs, greater than or equal to about 6MLs, greater than or equal to about 7 MLs, or greater than or equal toabout 8 MLs. In an embodiment, a thickness of the first semiconductornanocrystal shell may be greater than or equal to about 9 MLs, orgreater than or equal to about 10 MLs. A thickness of the firstsemiconductor nanocrystal shell may be less than about 5 nm. A thicknessof the first semiconductor nanocrystal shell may be less than or equalto about 15 MLs, less than or equal to about 14 MLs, or less than orequal to about 13 MLs.

The second semiconductor nanocrystal shell may include a zinc sulfide(e.g., ZnS). The second semiconductor nanocrystal shell may not includeselenium. The second semiconductor nanocrystal shell may disposeddirectly on the first semiconductor nanocrystal shell. A thickness ofthe second semiconductor nanocrystal shell may be selectedappropriately. The second semiconductor nanocrystal shell may be anoutermost layer of the quantum dots. In an embodiment, the quantum dotsmay have a core-multi shell structure wherein the quantum dots include acore including an indium phosphide (e.g., InP or InZnP), a first shelldisposed directly on the core and including ZnSe, and a second shelldisposed directly on the first shell and including ZnS.

In an embodiment, the quantum dots may include an InP core (e.g., mayemit red light) and an amount of the indium with respect to thechalcogen element may be in a predetermined range.

In the quantum dots of an embodiment, a molar ratio of indium to a sumof sulfur and selenium (i.e., [In:(Se+S)], hereinafter which may berecited as the value of [In/(Se+S)]) may be less than 0.1:1, less thanor equal to about 0.09:1, less than or equal to about 0.08:1, less thanor equal to about 0.07:1, less than or equal to about 0.06:1, or lessthan or equal to about 0.05:1. The molar ratio of indium to a sum ofsulfur and selenium may be greater than or equal to about 0.02:1,greater than or equal to about 0.03:1, or greater than or equal to about0.04:1.

In the quantum dots of an embodiment, a molar ratio of sulfur toselenium (S/Se) may be less than 4.5:1, for example, less than or equalto about 4:1, less than or equal to about 3.5:1, less than or equal toabout 3:1, less than or equal to about 2.5:1, less than or equal toabout 2:1, less than or equal to about 1.5:1, or less than or equal toabout 1:1. In the quantum dots of an embodiment, a molar ratio of sulfurto selenium (S/Se) may be greater than or equal to about 0.01:1, greaterthan or equal to about 0.05:1, greater than or equal to about 0.09:1,greater than or equal to about 0.1:1, greater than or equal to about0.11:1, greater than or equal to about 0.12:1, greater than or equal toabout 0.13:1, greater than or equal to about 0.14:1, greater than orequal to about 0.15:1, greater than or equal to about 0.16:1, greaterthan or equal to about 0.17:1, greater than or equal to about 0.18:1,greater than or equal to about 0.19:1, greater than or equal to about0.20:1, greater than or equal to about 0.3:1, greater than or equal toabout 0.4:1, greater than or equal to about 0.5:1, greater than or equalto about 0.6:1, greater than or equal to about 0.7:1, greater than orequal to about 0.8:1, or greater than or equal to about 0.9:1.

In the quantum dots, a molar ratio of zinc to indium may be greater thanor equal to about 15:1, greater than or equal to about 16:1, greaterthan or equal to about 17:1, greater than or equal to about 18:1,greater than or equal to about 19:1, greater than or equal to about20:1, greater than or equal to about 21:1, or greater than or equal toabout 22:1.

In the quantum dots, a molar ratio of zinc to indium may be less thanabout 52:1, less than or equal to about 51:1, less than or equal toabout 50:1, less than or equal to about 49:1, less than or equal toabout 48:1, or less than or equal to about 47:1.

A shape of the quantum dots is not particularly limited, and may, forexample, be a spherical, polyhedron, pyramid, multipod, or cube shape,nanotube, nanowire, nanofiber, nanosheet, or a combination thereof, butis not limited thereto.

The quantum dots may include the organic ligand, the organic solvent, ora combination thereof, which will be described below, on a surface ofthe quantum dots. The organic ligand, the organic solvent, or acombination thereof may be bound to the surface of the quantum dot.

In an embodiment, a method of producing the aforementioned quantum dotsincludes: providing the aforementioned semiconductor nanocrystal core;and forming a shell on the core.

The formation of the shell may include:

heating a first mixture including a first shell precursor containingzinc, an organic ligand, and an organic solvent;

adding a semiconductor nanocrystal core (for example, not in a heatedstate) including indium and phosphorous to the heated first mixture,

heating the first mixture to a first reaction temperature and adding aselenium containing precursor to the first mixture to conduct a reactionfor a predetermined time period,

increasing a temperature of a resulting mixture to a second reactiontemperature and adding a selenium containing precursor and a sulfurcontaining precursor separately to conduct a reaction.

The method may further include adding the first shell precursor to theresulting mixture.

Amounts of the selenium containing precursor and the sulfur containingprecursor with respect to the core (and optionally the added amount ofthe precursors, the temperature of adding the precursor, the reactiontemperature and time, or a combination thereof) may be controlled inorder to obtain a desired composition and size (e.g., in theaforementioned ranges) of the quantum dots and the properties thereof(e.g., the Stokes shift).

Details of the quantum dots are the same as set forth above.

The first shell precursor is not particularly limited and may beselected appropriately. In an embodiment, the first shell precursor mayinclude a Zn metal powder, an alkylated Zn compound, a Zn alkoxide, a Zncarboxylate, a Zn nitrate, a Zn perchlorate, a Zn sulfate, a Znacetylacetonate, a Zn halide, a zinc carbonate, a Zn cyanide, a Znhydroxide, a Zn oxide, a Zn peroxide, or a combination thereof. Examplesof the first shell precursor may include dimethyl zinc, diethyl zinc,zinc acetate, zinc acetylacetonate, zinc iodide, zinc bromide, zincchloride, zinc fluoride, zinc carbonate, zinc cyanide, zinc nitrate,zinc oxide, zinc peroxide, zinc perchlorate, zinc sulfate, and the like.One shell precursor or a plurality of different first shell precursorsmay be used.

In an embodiment, the organic ligand may include RCOOH, RNH₂, R₂NH, R₃N,RSH, RH₂PO, R₂HPO, R₃PO, RH₂P, R₂HP, R₃P, ROH, RCOOR′, RPO(OH)₂, RHPOOH,RHPOOH (wherein R and R′ are the same or different, and are eachindependently a C1 to C40 (e.g., C3 to C24) aliphatic hydrocarbon groupsuch as an alkyl, an alkenyl, or an alkynyl group or a C6 to C20aromatic hydrocarbon group such as an aryl group such as a phenylgroup), a polymeric organic ligand, or a combination thereof.

The organic ligand may coordinate, e.g., bind, to the surface of theobtained nanocrystal, and may allow the nanocrystal to be well dispersedin the solution, have an effect on the light emitting and electricalproperties of the quantum dot, or a combination thereof.

Examples of the organic ligand compound may include:

a thiol compound such as methane thiol, ethane thiol, propane thiol,butane thiol, pentane thiol, hexane thiol, octane thiol, dodecane thiol,hexadecane thiol, octadecane thiol, benzyl thiol, or a combinationthereof;

an amine compound such as methylamine, ethylamine, propylamine,butylamine, pentylamine, hexylamine, octylamine, nonylamine, decylamine,dodecylamine, hexadecylamine, octadecylamine, dimethylamine,diethylamine, dipropylamine, tributylamine, trioctylamine, or acombination thereof;

a carboxylic acid compound such as methanoic acid, ethanoic acid,propanoic acid, butanoic acid, pentanoic acid, hexanoic acid, heptanoicacid, octanoic acid, dodecanoic acid, hexadecanoic acid, octadecanoicacid, oleic acid, benzoic acid, or a combination thereof;

a phosphine compound such as methyl phosphine, ethyl phosphine, propylphosphine, butyl phosphine, pentyl phosphine, octyl phosphine, dioctylphosphine, tributyl phosphine, trioctyl phosphine, or a combinationthereof;

a phosphine oxide compound such as methyl phosphine oxide, ethylphosphine oxide, propyl phosphine oxide, butyl phosphine oxide, pentylphosphine oxide, tributyl phosphine oxide, octyl phosphine oxide,dioctyl phosphine oxide, trioctyl phosphine oxide, or a combinationthereof;

diphenyl phosphine, triphenyl phosphine, or an oxide compound thereof,or a combination thereof;

a mono- or di(C5 to C20 alkyl)phosphinic acid such as mono- or dihexylphosphinic acid, mono- or dioctyl phosphinic acid, mono- or didodecylphosphinic acid, mono- or di(tetradecyl)phosphinic acid, mono- ordi(hexadecyl)phosphinic acid, mono- or di(octadecyl)phosphinic acid, ora combination thereof;

a C5 to C20 alkyl phosphonic acid such as hexyl phosphonic acid, octylphosphonic acid, dodecyl phosphonic acid, tetradecyl phosphonic acid,hexadecyl phosphonic acid, octadecyl phosphonic acid, or a combinationthereof;

or a combination thereof.

One or more organic ligands may be used.

Examples of the organic solvent may include a C6 to C22 primary aminesuch as a hexadecylamine, a C6 to C22 secondary amine such asdioctylamine, a C6 to C40 tertiary amine such as a trioctyl amine, anitrogen-containing heterocyclic compound such as pyridine, a C6 to C40olefin such as octadecene, a C6 to C40 aliphatic hydrocarbon such ashexadecane, octadecane, squalene, or squalane, an aromatic hydrocarbonsubstituted with a C6 to C30 alkyl group such as phenyldodecane,phenyltetradecane, or phenyl hexadecane, a primary, secondary, ortertiary phosphine (e.g., trioctyl phosphine) containing at least one(e.g., 1, 2, or 3) C6 to C22 alkyl group, a phosphine oxide (e.g.,trioctyl phosphine oxide) containing at least one (e.g., 1, 2, or 3) C6to C22 alkyl group, a C12 to C22 aromatic ether such as a phenyl etheror a benzyl ether, or a combination thereof.

The solvent and amount of the solvent used may be selected taking intoconsideration the precursor and organic ligands used.

The first mixture may be heated to a temperature of greater than orequal to about 100° C., greater than or equal to about 120° C., greaterthan or equal to about 150° C., greater than or equal to about 200° C.,greater than or equal to about 250° C., or greater than or equal toabout 270° C., for example, under vacuum, an inert atmosphere, or acombination thereof for a predetermined time period (e.g., greater thanor equal to about 5 minutes and less than or equal to about 1 hour(hr)).

Details of the semiconductor nanocrystal core including indium andphosphorous are the same as set forth above. The core may becommercially available or may be synthesized in a suitable method. Amethod of preparing the core is not particularly limited and a suitablemethod of producing an indium phosphide-based core may be used. In anembodiment, the core may be formed by a hot injection manner wherein asolution including a metal precursor (e.g., an indium precursor) andoptionally a ligand is heated to a high temperature (e.g., of greaterthan or equal to about 200° C.) and a phosphorous precursor is injectedto the hot solution.

The selenium containing precursor is not particularly limited and may beselected appropriately. For example, the selenium containing precursormay include selenium-trioctyl phosphine (Se-TOP), selenium-tributylphosphine (Se-TBP), selenium-triphenyl phosphine (Se-TPP), or acombination thereof, but is not limited thereto.

The first reaction temperature may be greater than or equal to about280° C., greater than or equal to about 290° C., greater than or equalto about 300° C., greater than or equal to about 310° C., or greaterthan or equal to about 315° C. and less than or equal to about 350° C.,less than or equal to about 340° C., or less than or equal to about 330°C.

After or during the heating of the second mixture to the first reactiontemperature, a selenium containing precursor may be injected at leastone time (e.g., at least twice or at least three times).

The reaction mixture may be kept at the first reaction temperature for apredetermined time period (e.g., from 10 minutes to 60 minutes, from 15minutes to 55 minutes, or from 20 minutes to 50 minutes). Accordingly, afirst semiconductor nanocrystal shell including zinc and selenium may beformed to obtain a mixture including a particle of the aforementionedshell on the core.

Then, a temperature of the mixture may be raised to a second reactiontemperature, during which or when the mixture reaches the secondreaction temperature, a selenium containing precursor may be addedagain. The second reaction temperature may be greater than about thefirst reaction temperature. In an embodiment, the second reactiontemperature may be greater than the first reaction temperature by atleast about 10° C., at least about 20° C., or at least about 30° C. Thesecond reaction temperature may be controlled in a range of from about270° C. to about 350° C., from about 275° C. to about 345° C., fromabout 280° C. to about 340° C., from about 285° C. to about 335° C.,from about 290° C. to about 330° C., or from about 295° C. to about 325°C.

The first reaction temperature, the second reaction temperature, and thetime period at each of the reaction temperatures may be selected takinginto consideration a desired thickness of the shell including the firstsemiconductor nanocrystal, a desired thickness of the shell includingthe second semiconductor nanocrystal, the precursors, or the like. In anembodiment, the first and the second temperatures, the duration at eachof the reaction temperatures, and the amounts of the precursor may becontrolled such that the first semiconductor nanocrystal shell may havea thickness of the aforementioned range.

In the reaction system, an amount of the selenium as added per one moleof the indium may be greater than or equal to about 10 moles, greaterthan or equal to about 15 moles, greater than or equal to about 20moles, greater than or equal to about 25 moles, greater than or equal toabout 30 moles, greater than or equal to about 35 moles, greater than orequal to about 40 moles, or greater than or equal to about 45 moles andless than or equal to about 50 moles, less than or equal to about 45moles, less than or equal to about 40 moles, less than or equal to about35 moles, less than or equal to about 30 moles, less than or equal toabout 25 moles, or less than or equal to about 20 moles, but is notlimited thereto. The amount of the selenium may be selected taking intoconsideration the reaction temperature/time, a desired thickness of thefirst semiconductor nanocrystal shell, the selenium containingprecursor, or a combination thereof.

The injection of the sulfur precursor may begin after (for example, theentire amount of) the selenium containing precursor as added is used(consumed) in the reaction mixture.

In an embodiment, the method may not include decreasing the temperatureof the reaction mixture (for example, at or below 100° C., at or below50° C., or at or below 30° C., or at room temperature).

The sulfur containing precursor is not particularly limited and may beselected appropriately. The sulfur containing precursor may includehexane thiol, octane thiol, decane thiol, dodecane thiol, hexadecanethiol, mercapto propyl silane, sulfur-trioctyl phosphine (S-TOP),sulfur-tributyl phosphine (S-TBP), sulfur-triphenyl phosphine (S-TPP),sulfur-trioctylamine (S-TOA), trimethylsilyl sulfide, ammonium sulfide,sodium sulfide, or a combination thereof. The sulfur containingprecursor may be injected at least on time (e.g., at least twice).

In the reaction system, the amount of the sulfur containing precursorwith respect to the indium of the core may be selected taking intoconsideration a desired composition of the resulting quantum dots, areactivity of the sulfur containing precursor, and the second reactiontemperature.

In an embodiment, the an amount of the sulfur as added per one mole ofthe indium may be greater than or equal to about 1 moles, greater thanor equal to about 3 moles, greater than or equal to about 5 moles,greater than or equal to about 7 moles, greater than or equal to about 9moles, greater than or equal to about 11 moles, greater than or equal toabout 13 moles, or greater than or equal to about 15 moles and less thanor equal to about 30 moles, less than or equal to about 25 moles, lessthan or equal to about 20 moles, less than or equal to about 19 moles,less than or equal to about 18 moles, less than or equal to about 17moles, less than or equal to about 16 moles, less than or equal to about15 moles, or less than or equal to about 14 moles, but is not limitedthereto.

When a non-solvent is added into a resulting reaction solution asobtained, organic ligand-coordinated quantum dots may be separated(e.g., precipitated). The non-solvent may be a polar solvent that ismiscible with the solvent used in the reaction and nanocrystals are notdispersible therein. The non-solvent may be selected depending on thesolvent used in the reaction and may be for example, acetone, ethanol,butanol, isopropanol, ethanediol, water, tetrahydrofuran (THF), dimethylsulfoxide (DMSO), diethylether, formaldehyde, acetaldehyde, but is notlimited thereto. The separation may be performed through acentrifugation, precipitation, chromatography, or distillation. Theseparated nanocrystal may be added to a washing solvent and washed, ifdesired. The washing solvent is not particularly limited and may includea solvent having a similar solubility parameter to that of the ligandand may, for example, include hexane, heptane, octane, chloroform,toluene, benzene, and the like.

In an embodiment, the emissive layer 13 may have a thickness of greaterthan or equal to about 5 nm, greater than or equal to about 6 nm,greater than or equal to about 7 nm, greater than or equal to about 8nm, greater than or equal to about 9 nm, greater than or equal to about10 nm, greater than or equal to about 11 nm, greater than or equal toabout 12 nm, greater than or equal to about 13 nm, greater than or equalto about 14 nm, greater than or equal to about 15 nm, greater than orequal to about 16 nm, greater than or equal to about 17 nm, greater thanor equal to about 18 nm, greater than or equal to about 19 nm, orgreater than or equal to about 20 nm. The thickness of the emissivelayer 13 may be less than or equal to about 200 nm, less than or equalto about 190 nm, less than or equal to about 180 nm, less than or equalto about 170 nm, less than or equal to about 160 nm, less than or equalto about 150 nm, less than or equal to about 140 nm, less than or equalto about 130 nm, less than or equal to about 120 nm, less than or equalto about 110 nm, less than or equal to about 100 nm, less than or equalto about 90 nm, less than or equal to about 80 nm, less than or equal toabout 70 nm, less than or equal to about 60 nm, less than or equal toabout 50 nm, less than or equal to about 40 nm, less than or equal toabout 30 nm, or less than or equal to about 20 nm.

In an embodiment, the emissive layer 13 may have a HOMO energy levelthat is greater than or equal to about 5.4 eV, greater than or equal toabout 5.5 eV, greater than or equal to about 5.6 eV, greater than orequal to about 5.7 eV, greater than or equal to about 5.8 eV, greaterthan or equal to about 5.9 eV, or greater than or equal to about 6.0 eV.The HOMO energy level of the emissive layer 13 may be less than or equalto about 7.0 eV, less than or equal to about 6.8 eV, less than or equalto about 6.7 eV, less than or equal to about 6.5 eV, less than or equalto about 6.3 eV, or less than or equal to about 6.2 eV. The HOMO energylevel of the emissive layer 13 may be from about 5.4 eV to about 5.9 eV.

In an embodiment, the emissive layer 13 may have a LUMO energy levelthat is less than or equal to about 3.9 eV, less than or equal to about3.8 eV, less than or equal to about 3.7 eV, less than or equal to about3.6 eV, less than or equal to about 3.5 eV, less than or equal to about3.4 eV, less than or equal to about 3.3 eV, less than or equal to about3.2 eV, or less than or equal to about 3.0 eV. The LUMO energy level ofthe emissive layer 13 may be greater than or equal to about 2.5 eV,greater than or equal to about 2.6 eV, greater than or equal to about2.7 eV, or greater than or equal to about 2.8 eV. In an embodiment, theemissive layer 13 may have an energy bandgap of from about 2.4 eV toabout 3.5 eV.

In an embodiment, a hole auxiliary layer 12 may be disposed between thefirst electrode 11 (e.g., anode) and the emission layer 13. The holeauxiliary layer 12 may have one layer or two or more layers, and mayinclude, for example, a hole injection layer (HIL), a hole transportlayer (HTL), an electron blocking layer, or a combination thereof.

The hole auxiliary layer 12 may have a HOMO energy level that may matcha HOMO energy level of the emission layer 13 and may enforce, e.g., aid,mobility of holes from the hole auxiliary layer 12 into the emissionlayer 13.

The HOMO energy level of the hole auxiliary layer 12 (e.g., holetransport layer (HTL)) contacting the emission layer may be the same asor less than the HOMO energy level of the emission layer 13 by a valuewithin a range of less than or equal to about 1.0 eV, for example, fromabout 0.01 eV to about 0.8 eV, from about 0.01 eV to about 0.7 eV, fromabout 0.01 eV to about 0.5 eV, from about 0.01 eV to about 0.4 eV, fromabout 0.01 eV to about 0.3 eV, from about 0.01 eV to about 0.2 eV, orfrom about 0.01 eV to about 0.1 eV.

The HOMO energy level of the hole auxiliary layer 12 may be greater thanor equal to about 5.0 eV, greater than or equal to about 5.2 eV, greaterthan or equal to about 5.4 eV, greater than or equal to about 5.6 eV, orgreater than or equal to about 5.8 eV. In an embodiment, the HOMO energylevel of the hole auxiliary layer 12 may be from about 5.0 eV to about7.0 eV, from about 5.2 eV to about 6.8 eV, from about 5.4 eV to about6.8 eV, from about 5.4 eV to about 6.7 eV, from about 5.4 eV to about6.5 eV, from about 5.4 eV to about 6.3 eV, from about 5.4 eV to about6.2 eV, from about 5.4 eV to about 6.1 eV, from about 5.6 eV to about7.0 eV, from about 5.6 eV to about 6.8 eV, from about 5.6 eV to about6.7 eV, from about 5.6 eV to about 6.5 eV, from about 5.6 eV to about6.3 eV, from about 5.6 eV to about 6.2 eV, from about 5.6 eV to about6.1 eV, from about 5.8 eV to about 7.0 eV, from about 5.8 eV to about6.8 eV, from about 5.8 eV to about 6.7 eV, from about 5.8 eV to about6.5 eV, from about 5.8 eV to about 6.3 eV, from about 5.8 eV to about6.2 eV, or from about 5.8 eV to about 6.1 eV.

In an embodiment, the hole auxiliary layer 12 may include a holeinjection layer nearer to the first electrode 11 and a hole transportlayer nearer to the emission layer 13. In an embodiment, the HOMO energylevel of the hole injection layer may be from about 5.0 eV to about 6.0eV, about 5.0 eV to about 5.5 eV, about 5.0 eV to about 5.4 eV. In anembodiment, the HOMO energy level of the hole transport layer may befrom about 5.2 eV to about 7.0 eV, from about 5.4 eV to about 6.8 eV,from about 5.4 eV to about 6.7 eV, from about 5.4 eV to about 6.5 eV,from about 5.4 eV to about 6.3 eV, from about 5.4 eV to about 6.2 eV, orfrom about 5.4 eV to about 6.1 eV.

A material included in the hole auxiliary layer 12 (for example, a holetransporting layer or a hole injection layer) is not particularlylimited and may include, for example,poly(9,9-dioctyl-fluoren-2,7-diyl-co-N-(4-butylphenyl)-diphenylamine)(TFB), polyarylamine, poly(N-vinylcarbazole),poly(3,4-ethylenedioxythiophene) (PEDOT),poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS),polyaniline, polypyrrole, N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine(TPD), 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD), m-MTDATA(4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine),4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA),1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), a p-type metal oxide(e.g., NiO, WO₃, MoO₃, etc.), a carbon-based material such as grapheneoxide, or a combination thereof, but is not limited thereto.

The electron blocking layer (EBL) may include, for example,poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS),poly(9,9-dioctyl-fluoren2,7-diyl-co-N-(4-butylphenyl)-diphenylamine)(TFB) polyarylamine, poly(N-vinylcarbazole), polyaniline, polypyrrole,N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (TPD),4,4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD), m-MTDATA,4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA), or a combinationthereof, but is not limited thereto.

In the hole auxiliary layer(s), a thickness of each layer may beappropriately selected for example, depending on the desiredcharacteristics of each layer. For example, the thickness of each layermay be greater than or equal to about 10 nm, for example, greater thanor equal to about 15 nm, greater than or equal to about 20 nm and lessthan or equal to about 100 nm, for example, less than or equal to about90 nm, less than or equal to about 80 nm, less than or equal to about 70nm, less than or equal to about 60 nm, less than or equal to about 50nm, less than or equal to about 40 nm, less than or equal to about 35nm, or less than or equal to about 30 nm, but is not limited thereto.

The electron auxiliary layer 14 is disposed between the emissive layer13 and the second electrode 15. The electron auxiliary layer 14 mayinclude, for example, an electron injection layer (EIL) facilitating theinjection of the electrons, an electron transport layer (ETL)facilitating the transport of the electrons, a hole blocking layer (HBL)blocking the hole movement, or a combination thereof, but is not limitedthereto.

In an embodiment, the EIL may be disposed between the ETL and thecathode. In an embodiment, the HBL may be disposed between the ETL (orthe EIL) and the emissive layer, but is not limited thereto. In anembodiment, a thickness of each layer may be greater than or equal toabout 1 nm and less than or equal to about 500 nm, but is not limitedthereto. The EIL may be an organic layer (e.g., prepared by vapordeposition). The ETL may include an inorganic oxide nanoparticle, anorganic layer (e.g., prepared by vapor deposition), or a combinationthereof.

The electron transport layer, the electron injection layer, or acombination thereof may include, for example,1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), bathocuproine(BCP), tris[3-(3-pyridyl)-mesityl]borane (3TPYMB), LiF, Alq₃, Gaq₃,Inq₃, Znq₂, Zn(BTZ)₂, BeBq₂, ET204(8-(4-(4,6-di(naphthalen-2-yl)-1,3,5-triazin-2-yl)phenyl)quinolone),8-hydroxyquinolinato lithium (Liq), an n-type metal oxide (e.g., ZnO,HfO₂, etc.), or a combination thereof, but is not limited thereto.

The hole blocking layer (HBL) may include, for example,1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), bathocuproine(BCP), tris[3-(3-pyridyl)-mesityl]borane (3TPYMB), LiF, Alq₃, Gaq₃,Inq₃, Znq₂, Zn(BTZ)₂, BeBq₂, or a combination thereof, but is notlimited thereto.

In an embodiment, the electron auxiliary layer 14 (e.g., the electrontransport layer) may include a plurality of nanoparticles. Thenanoparticles include a metal oxide including zinc (e.g., a zinc metaloxide). In a device of an embodiment, the quantum dots having theaforementioned features (e.g., the composition, the size, the Stokesshift, or the like) may achieve desirable electroluminescent properties(e.g., a high level of EQE and an increased luminance) when combinedwith an electron auxiliary layer based on, e.g., including, a zinc metaloxide. In addition, when combined with an electron auxiliary layer basedon, e.g., including, a zinc metal oxide, the device including theaforementioned quantum dots may exhibit improved stability (e.g., anincreased T₅₀).

The metal oxide may include zinc oxide, zinc magnesium oxide, or acombination thereof. The metal oxide (e.g., the zinc metal oxide) mayinclude Zn_(1-x) M_(x)O (wherein M is Mg, Ca, Zr, W, Li, Ti, Y, Al, or acombination thereof and 0≤x≤0.5). In an embodiment, the M may bemagnesium (Mg). In an embodiment, the x may be zero. In an embodiment,the x may be greater than or equal to about 0.01, greater than or equalto about 0.02, greater than or equal to about 0.03, greater than orequal to about 0.04, greater than or equal to about 0.05, greater thanor equal to about 0.06, greater than or equal to about 0.07, greaterthan or equal to about 0.08, greater than or equal to about 0.09, orgreater than or equal to about 0.1 and less than or equal to about 0.45,less than or equal to about 0.4, less than or equal to about 0.35, lessthan or equal to about 0.3, less than or equal to about 0.25, less thanor equal to about 0.2, or less than or equal to about 0.15.

An absolute value of a LUMO energy level of quantum dots included in theemissive layer may be less than an absolute value of a LUMO energy levelof the metal oxide. In an embodiment, an absolute value of a LUMO energylevel of quantum dots may be greater than an absolute value of a LUMOenergy level of a metal oxide ETL.

An average size of the nanoparticles may be greater than or equal toabout 1 nm, for example, greater than or equal to about 1.5 nm, greaterthan or equal to about 2 nm, greater than or equal to about 2.5 nm, orgreater than or equal to about 3 nm and less than or equal to about 10nm, less than or equal to about 9 nm, less than or equal to about 8 nm,less than or equal to about 7 nm, less than or equal to about 6 nm, orless than or equal to about 5 nm. The nanoparticles may not have a rodshape. The nanoparticles may not have a nano wire shape.

In an embodiment, the thickness of the electron auxiliary layer 14(e.g., the thickness of each of an electron injection layer, an electrontransport layer, or a hole blocking layer) may be greater than or equalto about 5 nm, greater than or equal to about 6 nm, greater than orequal to about 7 nm, greater than or equal to about 8 nm, greater thanor equal to about 9 nm, greater than or equal to about 10 nm, greaterthan or equal to about 11 nm, greater than or equal to about 12 nm,greater than or equal to about 13 nm, greater than or equal to about 14nm, greater than or equal to about 15 nm, greater than or equal to about16 nm, greater than or equal to about 17 nm, greater than or equal toabout 18 nm, greater than or equal to about 19 nm, or greater than orequal to about 20 nm and less than or equal to about 120 nm, less thanor equal to about 110 nm, less than or equal to about 100 nm, less thanor equal to about 90 nm, less than or equal to about 80 nm, less than orequal to about 70 nm, less than or equal to about 60 nm, less than orequal to about 50 nm, less than or equal to about 40 nm, less than orequal to about 30 nm, or less than or equal to about 25 nm, but is notlimited thereto.

A device according to an embodiment has a normal structure. In a deviceaccording to an embodiment, an anode 11 a disposed on a transparentsubstrate 100 may include a metal oxide-based transparent electrode(e.g., ITO electrode) and a cathode 15 a facing the anode may include ametal (Mg, Al, etc.) of a relatively low work function. For example, ahole auxiliary layer 20, for example, a hole transport layer includingTFB, poly(9-vinylcarbazole) (PVK), or a combination thereof; a holeinjection layer including PEDOT:PSS, a p-type metal oxide, or acombination thereof; or a combination thereof may be disposed betweenthe anode 11 a, e.g., a transparent anode, and the emissive layer 30. Anelectron auxiliary layer 40 such as an electron injectionlayer/transport layer may be disposed between the quantum dot emissivelayer 30 and the cathode 15 a. (see FIG. 2)

A device according to an embodiment has an inverted structure. Herein,the cathode 15 a disposed on a transparent substrate 100 may include ametal oxide-based transparent electrode (e.g., ITO) and the anode 11 afacing the cathode may include a metal (e.g., Au, Ag, etc.) of arelatively high work function. For example, an n-type metal oxide (ZnO)may be disposed between the cathode 15 a, e.g., a transparent cathode,and the emissive layer 30 as an electron auxiliary layer 40 (e.g., anelectron transport layer (ETL)). MoO₃ or another p-type metal oxide as ahole auxiliary layer 20 (e.g., a hole transport layer (HTL) includingTFB, PVK, or a combination thereof; a hole injection layer (HIL)including MoO₃ or another p-type metal oxide; or a combination thereof)may be disposed between the metal anode 11 a and the quantum dotemissive layer 30 as a hole auxiliary layer (e.g., hole transport layer(HTL)). (see FIG. 3)

An embodiment is related to a method of preparing the aforementionedelectroluminescent device.

The method includes: forming an emissive layer including the quantumdots (e.g., a pattern of the aforementioned quantum dots) on a firstelectrode; optionally forming a charge auxiliary layer on the emissivelayer; and forming a second electrode on the emissive layer (oroptionally the charge auxiliary layer). The charge auxiliary layer maybe an electron auxiliary layer. The method may further include forming acharge auxiliary layer (e.g., a hole auxiliary layer) on the firstelectrode prior to the formation of the emissive layer. In this case,the emissive layer may be formed on the charge auxiliary layer disposedon the first electrode.

Details of the first electrode, the emissive layer, the charge auxiliarylayer, and the second electrode are the same as set forth above.

The formation of the electrode/hole auxiliary layer/electron auxiliarylayer is not particularly limited and may be selected appropriatelytaking into consideration the material, the thickness of theelectrode/layer to prepare, or the like. The formation may be carriedout via a solution process, a (physical or chemical) deposition process,or a combination thereof.

Forming the emissive layer may be carried out by obtaining a compositionincluding the quantum dots and an organic solvent, and applying ordepositing the composition on a substrate, an electrode, or a chargeauxiliary layer (e.g., through spin coating, inkjet printing, or contactprinting). The formation of the emissive layer may include heat-treatingthe applied or deposited quantum dot layer. A temperature for the heattreating is not particularly limited and may be selected appropriatelytaking into consideration a boiling point of the organic solvent. In anembodiment, the heat treating may be carried out at a temperature ofgreater than or equal to about 60° C. The organic solvent for thecomposition is not particularly limited and may be selectedappropriately. In an embodiment, the organic solvent may include a(substituted or unsubstituted) aliphatic hydrocarbon organic solvent, a(substituted or unsubstituted) aromatic hydrocarbon organic solvent, anacetate solvent, or a combination thereof.

Formation of the emissive layer may be carried out by preparing an inkcomposition including the aforementioned quantum dots of an embodimentand a liquid vehicle, and depositing the prepared ink composition (forexample, via an ink-jet printing method). Accordingly, an embodiment isrelated to an ink composition including the aforementioned quantum dotsand a liquid vehicle.

The ink composition may further include a light diffusing particle, abinder (e.g., a binder having a carboxylic acid group), and optionallyat least one additive (e.g., a photopolymerizable monomer (e.g., amonomer having a carbon-carbon double bond, a crosslinker, an initiator(e.g., photoinitiator), or a thiol compound, or the like). The lightdiffusing particle may include TiO₂, SiO₂, BaTiO₃, ZnO, or a combinationthereof. The light diffusing particle may have a size of greater than orequal to about 100 nm and less than or equal to about 1 μm.

The liquid vehicle may include an organic solvent. The organic solventmay include a hydrophilic (or water miscible) organic solvent. Theorganic solvent may include a hydrophobic organic solvent. The organicsolvent may include a polar (organic) solvent. The organic solvent mayinclude a non-polar (organic) solvent.

Types and amounts of the organic solvent may be selected appropriatelytaking into consideration the types and the amounts of theaforementioned main components (i.e., the quantum dot, the COOHgroup-containing binder, the photopolymerizable monomer, thecrosslinker, the initiator, and if used, the thiol compound).

Non-limiting examples of the liquid vehicle may include, but are notlimited to: ethyl 3-ethoxy propionate; an ethylene glycol series such asethylene glycol, diethylene glycol, or polyethylene glycol; a glycolether such as ethylene glycol monomethyl ether, ethylene glycolmonoethyl ether, diethylene glycol monomethyl ether, ethylene glycoldiethyl ether, or diethylene glycol dimethyl ether; a glycol etheracetate such as ethylene glycol monomethyl ether acetate, ethyleneglycol monoethyl ether acetate, diethylene glycol monoethyl etheracetate, or diethylene glycol monobutyl ether acetate; a propyleneglycol series such as propylene glycol; a propylene glycol ether such aspropylene glycol monomethyl ether, propylene glycol monoethyl ether,propylene glycol monopropyl ether, propylene glycol monobutyl ether,propylene glycol dimethyl ether, dipropylene glycol dimethyl ether,propylene glycol diethyl ether, or dipropylene glycol diethyl ether; apropylene glycol ether acetate such as propylene glycol monomethyl etheracetate or dipropylene glycol monoethyl ether acetate; an amide such asN-methylpyrrolidone, dimethyl formamide, or dimethyl acetamide; a ketonesuch as methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), orcyclohexanone; a petroleum product such as toluene, xylene, or solventnaphtha; an ester such as ethyl acetate, propyl acetate, butyl acetate,cyclohexyl acetate, or ethyl lactate; an ether such as diethyl ether,dipropyl ether, or dibutyl ether; a (unsubstituted or substituted, forexample, halogen or chloro substituted) aliphatic, alicyclic, oraromatic hydrocarbon or a carboxylate/ester derivative thereof (e.g.,cyclohexyl acetate or the like); or a combination thereof.

Components included in the ink composition and concentrations thereofmay be adjusted to control a viscosity of the ink composition, which isnot particularly limited. A viscosity of the ink composition may be lessthan or equal to about 20 centipoise (cP), less than or equal to about15 cP, less than or equal to about 10 cP, less than or equal to about 5cP, less than or equal to about 4 cP, less than or equal to about 3 cP,less than or equal to about 2 cP, or less than or equal to about 1.5 cP.The viscosity of the ink composition may be greater than or equal toabout 0.1 cP, greater than or equal to about 0.5 cP, or greater than orequal to about 0.8 cP.

The electroluminescent device that includes quantum dots of theembodiments may achieve an increased level of a EQE and an enhancedluminance. In an embodiment, the electroluminescent device may have anEQE that is greater than or equal to about 9%, greater than or equal toabout 9.5%, greater than or equal to about 10%, greater than or equal toabout 10.5%, greater than or equal to about 11%, or greater than orequal to about 11.5%. The EQE of the electroluminescent device may beless than or equal to about 30%, but is not limited thereto.

In an embodiment, the electroluminescent device may have a luminance ofgreater than or equal to about 10,000 cd/m², greater than or equal toabout 11,000 cd/m², greater than or equal to about 12,000 cd/m², greaterthan or equal to about 13,000 cd/m², greater than or equal to about14,000 cd/m², greater than or equal to about 15,000 cd/m², greater thanor equal to about 20,000 cd/m², greater than or equal to about 25,000cd/m², greater than or equal to about 30,000 cd/m², greater than orequal to about 35,000 cd/m², or greater than or equal to about 40,000cd/m². The Luminance of the electroluminescent device may be less thanor equal to about 500,000 cd/m², but is not limited thereto.

In an embodiment, the electroluminescent device may exhibit an extendedlifetime. In an embodiment, T50 of the electroluminescent device may begreater than about 10 hours, greater than or equal to about 15 hours,greater than or equal to about 20 hours, greater than or equal to about25 hours, or greater than or equal to about 30 hours.

In an embodiment, an electronic device includes the aforementionedquantum dots. The device may include a display device, a light emittingdiode (LED), an organic light emitting diode (OLED), a quantum dot LED,a sensor, a solar cell, an image sensor, or a liquid crystal display(LCD), but is not limited thereto. In an embodiment, the electronicdevice may include a photoluminescent device (e.g., a quantum dot sheetor a lighting such as a quantum dot rail or a liquid crystal display(LCD)). In an embodiment, the electronic device may include a quantumdot sheet and the aforementioned quantum dots are dispersed in the sheet(e.g., in the form of a quantum dot polymer composite).

Hereinafter, the embodiments are illustrated in more detail withreference to examples. However, these examples are exemplary, and thepresent scope is not limited thereto.

EXAMPLES Analysis Method 1. Ultraviolet-Visible (UV-Vis) SpectroscopicAnalysis

Hitachi U-3310 spectrometer is used to perform an ultraviolet (UV)spectroscopic analysis and obtain UV-Visible absorption spectra.

2. Photoluminescence Analysis

Photoluminescence (PL) spectra of the prepared nanocrystal are obtainedusing a Hitachi F-7000 spectrometer at an irradiation wavelength of 450nanometers (nm).

3. TEM Analysis

Transmission electron microscope (TEM) photographs of nanocrystals areobtained using an UT F30 Tecnai electron microscope.

4. X-Ray Diffraction (XRD) Analysis

An XRD analysis is performed using a Philips XPert PRO equipment with apower of 3 kilowatts (kW).

5. Electroluminescence Spectroscopic Analysis

A current depending on a voltage is measured using a Keithley 2635Bsource meter while applying a voltage and electroluminescent (EL) lightemitting luminance is measured using a CS2000 spectrometer.

Synthesis is performed under an inert gas atmosphere (nitrogen flowingcondition) unless particularly mentioned.

6. ICP Analysis

An inductively coupled plasma-atomic emission spectroscopy (ICP-AES)analysis is performed using Shimadzu ICPS-8100.

Reference Example 1: InP Cores are Prepared in the Following Manner

Indium acetate and palmitic acid are dissolved in 1-octadecene in a 200milliliters (mL) reaction flask, and heated under a vacuum state at 120°C. A molar ratio of indium to palmitic acid is 1:3. The atmosphere inthe flask is exchanged with N₂. After the reaction flask is heated to280° C., a mixed solution of tris(trimethylsilyl)phosphine (TMS₃P) andtrioctyl phosphine (TOP) is quickly injected, and the reaction proceedsfor 20 minutes. The reaction mixture then is rapidly cooled to roomtemperature and acetone is added thereto to produce nanocrystals, whichare then separated by centrifugation and dispersed in toluene to obtaina toluene dispersion of the InP core nanocrystals. The amount of theTMS₃P is about 0.5 moles per one mole of indium. A size of the InP corethus obtained is about 3.3 nm.

Reference Example 2: Synthesis of Zn Metal Oxide Nanoparticles

Zinc acetate dihydrate and magnesium acetate tetrahydrate are added intoa reactor including dimethyl sulfoxide to provide a molar ratioaccording to the following chemical formula and heated at 60° C. in anair atmosphere (mole ratio between zinc and magnesium=0.85:0.15).Subsequently, an ethanol solution of tetramethylammonium hydroxidepentahydrate is added into the reactor in a dropwise fashion at a speedof 3 milliliters (mL) per minute (mL/min). After stirring the same, theobtained zinc magnesium oxide (hereinafter, ZnMgO) nanoparticles arecentrifuged and dispersed in ethanol.

The obtained nanoparticles are subjected to an X-ray diffractionanalysis, so it is confirmed that they include a crystalline structure.The obtained nanoparticles are analyzed by a transmission electronmicroscopic analysis, and the results show that the particles have anaverage size of about 3 nm.

Example 1

Selenium and sulfur are dispersed in trioctyl phosphine (TOP) to obtaina Se/TOP stock solution and a S/TOP stock solution, respectively.

In a 200 mL reaction flask, zinc acetate and oleic acid are dissolved intrioctyl amine and the solution is subjected to vacuum at 120° C. for 10minutes. The atmosphere in the flask is replaced with N₂. While theresulting solution is heated to about 180° C., a toluene dispersion ofthe InP semiconductor nanocrystal core is injected thereto.

A resulting mixture is heated to 280° C. and the Se/TOP stock solutionis injected into the reaction flask and a reaction is carried out andthen the reaction temperature is raised again to 320° C., and the Se/TOPstock solution is injected again to carry out the reaction, therebyforming a ZnSe shell on the InP core. Then, the S/TOP stock solution isadded to the reaction system to carry out a reaction to form a ZnS shellon the ZnSe shell.

A total reaction time for forming the ZnSe shell is 60 minutes and atotal amount of the selenium used with respect to one mole of the indiumis about 20 moles. A total reaction time for forming the ZnS shell is 60minutes and a total amount of the sulfur used with respect to one moleof the indium is about 10 moles.

An excess amount of ethanol is added to the final reaction mixtureincluding the resulting InP/ZnSe/ZnS semiconductor nanocrystals, whichis then centrifuged. After centrifugation, the supernatant is discardedand the precipitate is dried and dispersed in chloroform to obtain aquantum dot solution (hereinafter, QD solution).

(2) For the obtained QD solution, an ICP-AES analysis is made and theresults are shown in Table 1. A UV-vis absorption spectroscopic analysisand a photoluminescence spectroscopic analysis are made for the QDsolution, and the results are shown in FIG. 4 and Table 1. A TEManalysis is made for the QDs to obtain an average particle size thereof.The results are shown in Table 1, together.

Example 2

InP/ZnSe/ZnS quantum dots are prepared in the same manner as set forthin Example 1 except that the amounts of the Se precursor and the Sprecursor used are 25 moles and 15 moles with respect to one mole of theindium, respectively.

For the obtained QD solution, an ICP-AES analysis is made and theresults are shown in Table 1. A UV-vis absorption spectroscopic analysisand a photoluminescence spectroscopic analysis are made for the QDsolution, and the results are shown in Table 1. A TEM analysis is madefor the QDs to obtain an average particle size thereof. The results areshown in Table 1, together.

Example 3

InP/ZnSe/ZnS quantum dots are prepared in the same manner as set forthin Example 1 except that the amounts of the Se precursor and the Sprecursor used are 30 moles and 20 moles with respect to one mole of theindium, respectively.

For the obtained QD solution, an ICP-AES analysis is made and theresults are shown in Table 1. A UV-vis absorption spectroscopic analysisand a photoluminescence spectroscopic analysis are made for the QDsolution, and the results are shown in Table 1. A TEM analysis is madefor the QDs to obtain an average particle size thereof. The results areshown in Table 1, together.

Comparative Example 1

InP/ZnSe/ZnS quantum dots are prepared in the same manner as set forthin Example 1 except that the amount of the Se precursor used is 20 moleswith respect to one mole of the indium and the temperature of formingthe ZnSe shell is 320° C.

For the obtained QD solution, an ICP-AES analysis is made and theresults are shown in Table 1. A UV-Vis absorption spectroscopic analysisand a photoluminescence spectroscopic analysis are made for the QDsolution, and the results are shown in Table 1. A TEM analysis is madefor the QDs to obtain an average particle size thereof. The results areshown in Table 1, together.

Comparative Example 2

InP/ZnSe/ZnS quantum dots are prepared in the same manner as set forthin Example 1 except that the amounts of the Se precursor and the Sprecursor used are 10 moles and 5 moles with respect to one mole of theindium, respectively, and the temperature of forming the ZnSe shell is320° C.

For the obtained QD solution, an ICP-AES analysis is made and theresults are shown in Table 1. A UV-Vis absorption spectroscopic analysisand a photoluminescence spectroscopic analysis are made for the QDsolution, and the results are shown in Table 1. A TEM analysis is madefor the QDs to obtain an average particle size thereof. The results areshown in Table 1, together.

TABLE 1 First UV average absorption Stokes particle peak PL peak shiftsize wavelength wavelength ICP-AES (nm) (nm) (nm) (nm) In:(Se + S) Com-35 10 594 629  0.05:1 parative Example 1 Com- 35 6 593 628 0.087:1parative Example 2 Example 1 16 9 612 628 0.042:1 Example 2 16 10 616632 0.034:1 Example 3 14 12 619 633 0.021:1

Stokes shift=PL wavelength(nm)−First UV absorption peak(nm)

Example 4

An electroluminescent device having the structure of indium tin oxide(ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)(35 nm)/poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine)(TFB) (25 nm)/quantum dot emissive layer (20 nm)/ZnMgO (40 nm)/Al (100nm) is prepared as below.

An ITO-deposited glass substrate is surface-treated with UV-ozone for 15minutes and then spin-coated with a PEDOT:PSS solution (H. C. Starks)and heated at 150° C. for 10 minutes under an air atmosphere, and thenheat-treated again at 150° C. for 10 minutes under an N₂ atmosphere toprovide a hole injection layer having a thickness of 35 nm.

Subsequently, apoly[(9,9-dioctylfluorenyl-2,7-diyl-co(4,4′-(N-4-butylphenyl)diphenylamine]solution (TFB) (Sumitomo) is spin-coated on the hole injection layer andheat-treated at 150° C. for 30 minutes to form a hole transport layer.

The core/shell quantum dots obtained from Example 1 is spin-coated onthe hole transport layer and heat-treated at 80° C. for 30 minutes toform a quantum dot emissive layer.

A solution of the ZnMgO nanoparticles according to Reference Example 2(a solvent: ethanol, optical density: 0.5 a.u) is prepared. The solutionis spin-coated on the second layer and heat-treated at 80° C. for 30minutes to form an electron auxiliary layer. On the surface of theelectron auxiliary layer, aluminum (Al) is vacuum-deposited to form asecond electrode.

Electroluminescent properties of the obtained quantum dot light emittingdevice are evaluated using a Keithley 2200 source measuring device and aMinolta CS2000 spectroradiometer (current-voltage-luminance measurementequipment). A current depending upon a voltage applied to the device,luminance, and electroluminescence (EL) are measured by thecurrent-voltage-luminance measurement equipment, and thereby externalquantum efficiency is calculated. The results are shown in Table 2, andFIGS. 5 and 6.

Example 5

An electroluminescent device is manufactured in the same manner as setforth in Example 4, except for using the quantum dots of Example 2.

Electroluminescent properties of the obtained quantum dot light emittingdevice are evaluated using a Keithley 2200 source measuring device and aMinolta CS2000 spectroradiometer (current-voltage-luminance measurementequipment). A current depending upon a voltage applied to the device,luminance, and electroluminescence (EL) are measured by thecurrent-voltage-luminance measurement equipment, and thereby externalquantum efficiency is calculated. The results are shown in Table 2.

Comparative Example 3

An electroluminescent device is manufactured in the same manner as setforth in Example 4, except for using the quantum dots of ComparativeExample 2.

Electroluminescent properties of the obtained quantum dot light emittingdevice are evaluated using a Keithley 2200 source measuring device and aMinolta CS2000 spectroradiometer (current-voltage-luminance measurementequipment). A current depending upon a voltage applied to the device,luminance, and electroluminescence (EL) are measured by thecurrent-voltage-luminance measurement equipment, and thereby externalquantum efficiency is calculated. The results are shown in Table 2.

Comparative Example 4

An electroluminescent device is manufactured in the same manner as setforth in Example 4, except that the quantum dots of Comparative Example2 are used and a triaryltriazine compound is used to form an organic ETLinstead of the zinc metal oxide particle.

Electroluminescent properties of the obtained quantum dot light emittingdevice are evaluated using a Keithley 2200 source measuring device and aMinolta CS2000 spectroradiometer (current-voltage-luminance measurementequipment). A current depending upon a voltage applied to the device,luminance, and electroluminescence (EL) are measured by thecurrent-voltage-luminance measurement equipment, and thereby externalquantum efficiency is calculated. The results are shown in Table 2

TABLE 2 Maximum Maximum external Luminance quantum (candelas efficiencyper square (EQE) meter T50 (hours (%) (cd/m²)) (h)) Comparative Example3 4.0 8000 10 (Comparative Example 2 QD + ZnMgO ETL) Example 4 11.915000 30 (Example 1 QD + ZnMgO ETL) Example 5(Example 2 QD + 10.0 1800050 ZnMgO ETL) Comparative Example 4 1.6 950 0.3 (Comparative Example 2QD + organic ETL)

T50 lifetime (h): at the operation of the device at 100 nits (cd/m²),time taken for which the luminance of the device is reduced to 50% ofits initial value (100%)

The results of Table 2 confirm that the electroluminescent devices(including the quantum dots) of the Examples may exhibit improvedelectroluminescent properties and life time in comparison with thedevice of the Comparative Examples.

Example 6: (Example 1 QD+Organic ETL)

An electroluminescent device is manufactured in the same manner as setforth in Example 4, except that a triaryltriazine compound is used toform an organic ETL, instead of the zinc metal oxide particle.

Electroluminescent properties of the obtained quantum dot light emittingdevice are evaluated using a Keithley 2200 source measuring device and aMinolta CS2000 spectroradiometer (current-voltage-luminance measurementequipment). A current depending upon a voltage applied to the device,luminance, and electroluminescence (EL) are measured by thecurrent-voltage-luminance measurement equipment, and thereby externalquantum efficiency is calculated. The results confirm that the device ofExample 6 may exhibit enhanced EQE and luminance that are increased morethan 3 times over and more than 6 times over, respectively, incomparison with the device of Comparative Example 4.

Example 7: (Example 2 QD+Organic ETL)

An electroluminescent device is manufactured in the same manner as setforth in Example 4, except that the quantum dots of Example 2 are usedand a triaryltriazine compound is used to form an organic ETL, insteadof the zinc metal oxide particle.

Electroluminescent properties of the obtained quantum dot light emittingdevice are evaluated using a Keithley 2200 source measuring device and aMinolta CS2000 spectroradiometer (current-voltage-luminance measurementequipment). A current depending upon a voltage applied to the device,luminance, and electroluminescence (EL) are measured by thecurrent-voltage-luminance measurement equipment, and thereby externalquantum efficiency is calculated. The results confirm that the device ofExample 7 may exhibit enhanced EQE and luminance that are increased morethan 3 times over and more than 10 times over, respectively, incomparison with the device of Comparative Example 4.

While this disclosure has been described in connection with what ispresently considered to be practical example embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

DESCRIPTION OF SYMBOLS

-   -   10: light emitting Device    -   11: first electrode    -   11 a: anode    -   12, 20: hole auxiliary layer    -   13, 30: emissive layer    -   14, 40: electron auxiliary layer    -   15: second electrode    -   15 a: cathode    -   100: transparent substrate

What is claimed is:
 1. An electroluminescent device, comprising a firstelectrode and a second electrode facing each other, an emissive layerdisposed between the first electrode and the second electrode, theemissive layer comprising quantum dots, and wherein the quantum dotscomprise a semiconductor nanocrystal core comprising indium andphosphorous, a first semiconductor nanocrystal shell disposed on thesemiconductor nanocrystal core, the first semiconductor nanocrystalshell comprising zinc and selenium, and a second semiconductornanocrystal shell disposed on the first semiconductor nanocrystal shell,the second semiconductor nanocrystal shell comprising zinc and sulfur,wherein the quantum dots do not comprise cadmium, wherein theelectroluminescent device has an external quantum efficiency of greaterthan or equal to about 9% and a maximum brightness of greater than orequal to about 10,000 candelas per square meter (cd/m²).
 2. Theelectroluminescent device of claim 1, wherein a wavelength of a maximumphotoluminescent peak of the quantum dots is present in a range ofgreater than or equal to about 600 nanometers and less than or equal toabout 650 nanometers.
 3. The electroluminescent device of claim 1,wherein an average particle size of the quantum dots is greater than orequal to about 8 nanometers.
 4. The electroluminescent device of claim1, wherein an average particle size of the quantum dots is greater thanor equal to about 9 nanometers.
 5. The electroluminescent device ofclaim 1, wherein a quantum yield of the quantum dots is greater than orequal to about 60%.
 6. The electroluminescent device of claim 1, whereina particle size distribution of the quantum dots is less than or equalto about 20%.
 7. The electroluminescent device of claim 1, wherein inthe quantum dots, a molar ratio of indium to a sum of sulfur andselenium In:(Se+S) is greater than or equal to about 0.02:1 and lessthan or equal to about 0.1:1.
 8. The electroluminescent device of claim1, wherein in the quantum dots, a molar ratio of sulfur to selenium S:Seis greater than or equal to about 0.01:1 and less than or equal to about4.5:1.
 9. The electroluminescent device of claim 1, wherein the firstsemiconductor nanocrystal shell is disposed directly on a surface of thesemiconductor nanocrystal core and the first semiconductor nanocrystalshell does not comprise sulfur.
 10. The electroluminescent device ofclaim 9, wherein a thickness of the first semiconductor nanocrystalshell is greater than or equal to about 6 monolayers.
 11. Theelectroluminescent device of claim 9, wherein the second semiconductornanocrystal shell is an outermost layer of the quantum dots and thesecond semiconductor nanocrystal shell is disposed directly on a surfaceof the first semiconductor nanocrystal shell.
 12. The electroluminescentdevice of claim 1, wherein a difference between a maximumphotoluminescent peak wavelength and a first absorption peak wavelengthof the quantum dots is less than or equal to about 16 nanometers. 13.The electroluminescent device of claim 1, wherein the electroluminescentdevice comprises an electron transport layer between the secondelectrode and the emissive layer.
 14. The electroluminescent device ofclaim 13, wherein the electron transport layer comprises a metal oxidecomprising zinc.
 15. The electroluminescent device of claim 14, whereinthe metal oxide comprises Zn_(1-x)M_(x)O, wherein M is Mg, Ca, Zr, W,Li, Ti, Y, Al, or a combination thereof and 0≤x≤0.5.
 16. Quantum dotscomprising a semiconductor nanocrystal core comprising indium andphosphorous, a first semiconductor nanocrystal shell disposed on thesemiconductor nanocrystal core, the first semiconductor nanocrystalshell comprising zinc and selenium, and a second semiconductornanocrystal shell disposed on the first semiconductor nanocrystal shell,the second semiconductor nanocrystal shell comprising zinc and sulfur,wherein the quantum dots do not comprise cadmium, and wherein adifference between a maximum photoluminescent peak wavelength and afirst absorption peak wavelength of the quantum dots is less than orequal to about 20 nanometers.
 17. The quantum dots of claim 16, whereinan average particle size of the quantum dots is greater than or equal toabout 8 nanometers and a particle size distribution of the quantum dotsis less than or equal to about 20% of the average particle size.
 18. Thequantum dots of claim 16, wherein in the quantum dots, a molar ratio ofindium to a sum of sulfur and selenium In:(Se+S) is greater than orequal to about 0.02:1 and less than or equal to about 0.1:1.
 19. Thequantum dots of claim 16, wherein the first semiconductor nanocrystalshell is disposed directly on a surface of the semiconductor nanocrystalcore and the first semiconductor nanocrystal shell does not comprisesulfur, and the second semiconductor nanocrystal shell is an outermostlayer of the quantum dots.
 20. The quantum dots of claim 19, wherein athickness of the first semiconductor nanocrystal shell is greater thanor equal to about 6 monolayers.
 21. The quantum dots of claim 16,wherein the difference between a maximum photoluminescent peakwavelength and a first absorption peak wavelength of the quantum dots isless than or equal to about 17 nanometers.
 22. The quantum dots of claim16, wherein an average particle size of the quantum dots is greater thanor equal to 8 nanometers and less than or equal to 50 nm.